About this replacement
Lenovo Z5s / L78071 — 3.85V Li-Polymer Replacement Battery (BL299)
This is a 3250mAh (12.51Wh) Li-Polymer cell replacing the original BL299 battery in the Lenovo Z5s (L78071). It fits the Z5s motherboard connector and BMS handshake directly. Install it when the original cell no longer holds a usable charge across a full day.
- Z5s and L78071 fitment: Both model designations share the same battery bay geometry, connector pinout, and BMS communication protocol — the L78071 is the hardware variant identifier for the same Z5s unit. One cell covers both.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on the Z5s platform. The BMS accepted the cell without error flags, and the protection circuit tripped correctly at the low-voltage cutoff threshold during deep discharge testing.
- Fuel gauge recalibration on first use: On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC map the new cell's discharge curve before high-current charging pushes current into an uncalibrated state — skipping this step is the main cause of erratic percentage readings.
Sudden shutdown at 20–30% on the Z5s after a cell swap
The Z5s fuel gauge IC holds a discharge model calibrated to the original cell. A new cell has a different internal resistance profile, so the IC misjudges the remaining capacity. Under modem or screen load, the cell voltage drops faster than the IC predicts, and the phone cuts out before the percentage reaches zero. One full discharge-charge cycle without fast charging resets the coulomb counter and realigns the model to the new cell. After that cycle, the shutdown threshold should stabilise above 5%.
USB-PD fast charge not accepted on the first cycle after replacement
Some Z5s units will not negotiate fast charge on the first full cycle with a new BL299 cell. The charge IC applies a trickle or standard rate until it reads a stable voltage signature from the new cell — this is normal BMS behaviour on a fresh cell. Charge once to 100% at standard rate, let it drain to around 10%, then charge again. Fast charge protocol typically re-engages on the second cycle once the IC has a confirmed voltage baseline above 3.6V.